We propose continued support for further development and maintenance of the `EEGLAB'software environment (http://sccn.ucsd.edu/eeglab), a now widely used open source software resource for analysis and visualization of brain dynamics from electrophysiological data. EEGLAB is designed to facilitate a major evolutionary step now taking place in the field of human electrophysiology. Long dominated by simple time-locked event-related potential (ERP) and field (ERF) averaging methods, human electroencephalographic (EEG), magnetoencephalographic (MEG), and invasive electrophysiological studies are increasingly making use of modern signal processing advances including time/frequency analysis, source localization onto magnetic resonance images of the head, three-dimensional and animated visualization methods, and spatial filtering including independent component analysis (ICA), applied not only to trial averages but directly to the high-dimensional single-trial data. Much yet unexploited information about human brain dynamics contained in such data is accessible to these methods. Yet, despite the ever-increasing affordability of high-speed computational resources that should allow every electrophysiological laboratory to perform sophisticated analyses on their data, the use of modern signal processing tools for these purposes is still rare, particularly for EEG analysis. The freely available and now widely used open source software toolbox EEGLAB, running under the widely used cross-platform MATLAB (The Mathworks, Inc.) programming environment, encompasses functions at three levels of user sophistication: a graphic user interface for exploratory signal processing, high-level and more low-level scripting methods for more experienced users, and a plug-in function facility that allows method innovators to distribute new or alternative analysis approaches to the EEG research community. In this continuation period, we propose incorporating further major technical improvements in data analysis and source localization, new methods to help users add new tools, new standardized distributed software development, documentation, and maintenance tools, and to continue building an open source user and developer community. Lay narrative for EEGLAB grant 5/15/07 This grant proposes continued development and maintenance of the widely used open source software environment for signal processing of human brain dynamic data collected as electroencephalography (EEG), magnetoencephalography (MEG), or for clinical purposes as invasive intracranial EEG (iEEG) data. EEGLAB, running on the very widely used Matlab software (The Mathworks, Inc., Natick MA), allows basic and clinical human brain researchers to apply new mathematical tools and modeling methods to their data, thereby increase the amount of information available in each study. Eventually, application of the new methods may lead to advances in neurology and psychiatry in the form of better diagnostic and monitoring tools, and better basic understanding of human brain function.